feat(mcp): add dual-era 2026-07-28 transport support

[Schreezer]

Summary

• add automatic MCP 2026-07-28 negotiation to the MCP client while retaining legacy 2025-era compatibility
• add a strict modern stateless server adapter for execute and skills
• route modern traffic through self-host, local HTTP, Cloud, and the standalone Cloudflare host without applying legacy session-id rules
• preserve live multi-round execution state in the Cloudflare Durable Object while carrying only signed continuation state on the client

Implementation

• keep @modelcontextprotocol/sdk v1 for the existing legacy server and MCP Apps surface
• add exact v2 client, core, and server packages only at the migration seams
• authenticate and validate Host before protocol-era classification; preserve the existing wildcard CORS contract while allowing hosts to enforce narrower origin policy at their own boundary
• use createMcpHandler(..., { legacy: "reject" }) for the modern path
• bind signed request state to account, organization, resource, method, tool, code digest, and execution id
• deduplicate concurrent resume calls, cache terminal results for idempotent retries, and reject cross-principal replay
• negotiate modern HTTP/SSE/stdio clients automatically and retain era-aware legacy session recovery behavior
• add MCP_2026_07_28_ENABLED=false as an inbound-only rollback switch; modern discovery returns a non-5xx protocol error so v2 auto negotiation can fall back to legacy
• single-flight Cloudflare dispatcher/runtime shutdown, lease active modern requests, and prevent idle alarms from tearing down active continuation state

Review loop

The same review sub-agent reviewed and re-reviewed the implementation through four rounds:

1. fixed rollback negotiation, concurrent first-request dispatcher creation, and cross-origin compatibility
2. fixed idle-alarm/runtime-shutdown races with active modern requests
3. fixed a lifecycle-gate deadlock found in the previous revision and replaced the helper test with a full-handler forced interleaving
4. no important actionable findings remained

Verification

• bun run lint
• bun run format:check
• @executor-js/host-mcp: typecheck and full package tests from the original implementation pass; focused envelope suite 12/12 after review fixes
• @executor-js/plugin-mcp: typecheck and 128 tests, 29 skipped live-server cases
• @executor-js/cloudflare: typecheck and full package tests from the original implementation pass; focused Durable Object suite 14/14 after review fixes
• self-host MCP: typecheck and 5 tests
• local MCP: typecheck and browser-resume regression test
• standalone Cloudflare host: typecheck and 13/13 real workerd tests, including modern stateless execution, durable input_required, and real v2-client rollback fallback to legacy

The workspace-wide typecheck is currently blocked in the untouched @executor-js/integrations-registry package by missing Node type declarations. The workspace-wide test run reached 30 successful packages before the MCP Apps browser suite failed because /usr/bin/google-chrome is unavailable in the environment.

Current draft boundary

• modern inbound clients currently receive the core execute and skills surface
• artifact tools and MCP Apps remain on the proven legacy server path pending a v2 extension migration
• the local inbound stdio server remains legacy; outbound stdio clients negotiate both eras
• existing OAuth behavior is preserved, but this PR does not claim a complete OAuth-spec audit for the 2026 revision

References

• https://ts.sdk.modelcontextprotocol.io/v2/protocol-versions
• https://ts.sdk.modelcontextprotocol.io/v2/migration/upgrade-to-v2
• https://ts.sdk.modelcontextprotocol.io/v2/migration/support-2026-07-28